2,3-Benzodiazepines
Certain 2,3-benzodiazepines have been explored extensively for their potent CNS modulating activity. Compounds such as tofisopam (Grandaxin®)(structure shown below, with the atom numbering system indicated), girisopam, and norisopam have demonstrated substantial anxiolytic and antipsychotic activity.

Tofisopam has been shown in humans to have an activity profile that is significantly different from that of widely used 1,4-benzodiazepine (BZ) anxiolytics such as diazepam (Valium®) and chlordiazepepoxide (Librium®). The 1,4-benzodiazepines, in addition to having sedative-hypnotic activity, also possess muscle relaxant and anticonvulsant properties that, though therapeutically useful in some disease states, are nonetheless potentially untoward side effects. Thus the 1,4-benzodiazepines, though safe when administered alone, may be dangerous in combination with other CNS drugs, including alcohol.
Tofisopam, in contrast, is a non-sedative anxiolytic that has no appreciable sedative, muscle relaxant or anticonvulsant properties (Horvath et al., Progress in Neurobiology, 60 (2000), 309-342). In clinical studies, tofisopam improved rather than impaired psychomotor performance and showed no interaction with ethanol (Id.). These observations comport with data that show that tofisopam does not interact with central BZ receptors and binds only weakly to peripheral BZ receptors.
Other 2,3-benzodiazepines that are structurally similar to tofisopam have been investigated and shown to have varying activity profiles. For example, GYKI-52466 and GYKI-53655 (structures shown below) act as noncompetitive glutamate antagonists at the AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) site, and have demonstrated neuroprotective, muscle relaxant and anticonvulsant activity (Id.). Another group of 2,3-benzodiazepines that have been investigated are represented by the compound GYKI-52895, and show activity as selective dopamine uptake inhibitors with potential use in antidepressant and anti-Parkinsonism therapy.

Tofisopam is a racemic mixture of (R)- and (S)-enantiomers. This is due to the asymmetric carbon, i.e., a carbon with four different groups attached, at the 5-position of the benzodiazepine ring.
The molecular structure and conformational properties of tofisopam have been determined by NMR, CD and x-ray crystallography (Visy et al., Chirality 1:271-275 (1989)). The 2,3-diazepine ring exists as two conformers. The major conformers, (+)R and (−)S have the 5-ethyl group in a quasi-equatorial position, while in the minor conformers, (−)R and (+)S, the 5-ethyl group is positioned quasi-axially. Thus, racemic tofisopam may exist as four molecular species, i.e., two enantiomers, each of which exists in two conformations. The sign of the optical rotation is reversed upon inversion of the diazepine ring from one conformer to the other. In crystal form, tofisopam exists only as the major conformations, with dextrorotatory tofisopam being of the (R) absolute configuration. (Toth et al., J. Heterocyclic Chem., 20:709-713 (1983); Fogassy et al., Bioorganic Heterocycles, Van der Plas, H. C., Ötvös, L, Simongi, M., eds. Budapest Amsterdam: Akademia; Kiado-Elsevier, 229:233 (1984)).
Differential binding of the (+) and (−) conformers of 2,3-benzodiazepines generally, has been reported for tofisopam in binding studies with human albumin (Simongi et al. Biochem. Pharm., 32(12), 1917-1920, 1983). The (+) and (−) conformers of tofisopam have also been reported as existing in an equilibrium (Zsila et al., Journal of Liquid Chromatography & Related Technologies, 22(5), 713-719, 1999; and references therein).
The optically pure (R)-enantiomer of tofisopam (R)-1-(3,4-dimethoxyphenyl)-4-methyl-5-ethyl-7,8-dimethoxy-5H-2,3-benzodiazepine) has been isolated and shown to possess the nonsedative anxiolytic activity of the racemic mixture. See U.S. Pat. No. 6,080,736; the entire disclosure of which is incorporated herein by reference.
Metabolism of Tofisopam
Tofisopam is metabolized in human, rat, dog, monkey and rabbit to one or more of six major metabolites, depending on the host species:
Compound #Compound Name11-(3,4-dimethoxyphenyl)-4-methyl-5-ethyl-7-hydroxy-8-methoxy-5H-2,3-benzodiazepine21-(3,4-dimethoxyphenyl)-4-methyl-5-ethyl-7-methoxy-8-hydroxy-5H-2,3-benzodiazepine31-(3-methoxy-4-hydroxyphenyl)-4-methyl-5-ethyl-7,8-dimethoxy-5H-2,3-benzodiazepine41-(3-hydroxy-4-methoxyphenyl)-4-methyl-5-ethyl-7,8-dimethoxy-5H-2,3-benzodiazepine51-(3-methoxy-4-hydroxyphenyl)-4-methyl-5-ethyl-7-hydroxy-8-methoxy-5H-2,3-benzodiazepine61-(3-hydroxy-4-methoxyphenyl)-4-methyl-5-ethyl-7-hydroxy-8-methoxy-5H-2,3-benzodiazepineSee Tomori et al., Journal of Chromatography, 241 (1982), p. 89-99.
Of the compounds named above, Compounds 1, 3 and 5 have been identified as metabolites in humans. These compounds have been synthesized and tested in certain pharmacological assays. C. Ito, “Behavioral Pharmacological Study on the Structure Activity Relationship of Benzodiazepine Derivatives: With Particular Reference to the Activity of 2,3-Benzodiazepine,” J. Tokyo Med. College, 39:369-384 (1981)._In an assay of inhibition of aggression in mice, Compound 1 and 3 showed 0% inhibition of aggression and Compound 5 showed a 28.6% inhibition of aggression. In an assay of muricide (mouse killing behavior) in rats, Compound 3 exhibited 0% inhibition of muricide while Compounds 1 and 5 each exhibited a 20% inhibition of muricide. In assays testing for anti-noradrenergic effects, Compound 1 exhibited no effect, while Compounds 3 and 5 demonstrated measurable activity.
Compounds 1, 3, 5 and 6 are also disclosed in U.S. Pat. No. 4,322,346, the entire disclosure of which is incorporated herein by reference. Compound 3 is reported therein to demonstrate narcosis-potentiating activity in mice.
Leukotriene B4 (LTB4)
Leukotrienes, along with prostaglandins and thromboxanes, are products of arachidonic acid metabolism. LTB4 is produced by leukocytes, particularly macrophage and monocytes upon activation by immune complexes, phagocytosis or other stimuli. LTB4 is a potent chemotactic agent that stimulates neutrophil and macrophage migration (chemotaxis) to sites of inflammation. The structure of LTB4 is shown below.

The known pathophysiological responses of LTB4 include: induction of potent neutrophil chemotactic activity, promotion of adhesion of polymorphonuclear leukocytes (PMN) to vasculature, increase in vascular permeability, stimulation of the release of lysosomal enzymes, by PMN. The pro-inflammatory action of LTB4 has been demonstrated in vivo, wherein topical LTB4 on human skin promotes the infiltration of PMN and other inflammatory cells. Intradermal injection of LTB4 induces accumulation of neutrophils at the injection site. Intravenous injection of LTB4 causes rapid but transient neutropenia (Kingsbury et al., J. Med. Chem., 1993, 36, 3308-3320; and references cited therein).
In addition, the presence of physiologically relevant LTB4 concentration at inflammatory sites has been associated with, for example, disease states such as psoriasis, asthma and active gout; in colonic mucosa associated with inflammatory bowel disease; in synovial fluid from patients with active rheumatoid arthritis; and in reperfusion injury. All of these observations together support the involvement of LTB4 in human inflammatory disease (Kingsbury et al, and Griffeths et al., Proc. Natl. Acad. Sci. Vol. 92, pp517-521, January 1995; and references cited therein.).
Inflammatory Disorders
Crohn's disease and ulcerative colitis, collectively referred to as inflammatory bowel disease (IBD), are chronic recurrent inflammatory diseases of unclear etiology, affecting the small intestine and colon. Inflammatory bowel disease (IBD) can involve either or both the small and large bowel. These disorders fall into the category of “idiopathic” inflammatory bowel disease because the etiology for them is unknown.
Pathologic findings are generally not specific, although they may suggest a particular form of IBD. “Active” IBD is characterized by acute inflammation. “Chronic” IBD is characterized by architectural changes of crypt distortion and scarring. The term “crypt” refers to a deep pit that protrudes down into the connective tissue surrounding the small intestine. Crypt abscesses (active IBD characterized by the presence of neutrophils in crypt lumens) can occur in many forms of IBD, not just ulcerative colitis. Under normal conditions the epithelium at the base of the crypt is the site of stem cell proliferation and the differentiated cells move upwards and are shed 3-5 days later at the tips of the villi. This normal process, necessary for proper bowel function, is interrupted by IBD
Ulcerative colitis (UC) involves the colon as a diffuse mucosal disease with distal predominance. The rectum is virtually always involved, and additional portions of colon may be involved extending proximally from the rectum in a continuous pattern. Most often ulcerative colitis occurs in young people 15 to 40 years of age. Ulcerative colitis occurs only in the inner lining of the colon (large intestine) or rectum. When it is localized in the rectum, it is called “proctitis.”
Crohn's Disease is a chronic inflammatory disease that has periods of remission (time when person feels well) and relapse (when a person feels ill). Crohn's disease is an inflammation and ulceration process that occurs in the deep layers of the intestinal wall. The most common areas affected are the lower part of the small intestine, called the ileum, and the first part of the colon. This type of Crohn's disease is called ileocolitis. Crohn's disease can infrequently affect any part of the upper gastrointestinal tract. Aphthous ulcers, which are similar to cold sores, are common. Ulcers can also occur in the esophagus, stomach and duodenum.
Therapy for IBD has historically included administration of corticosteroids. However drawbacks of long term corticosteroid therapy include masking (or induction) of intestinal perforation, osteonecrosis and metabolic bone disease. Additional problems relate to development of corticosteroid dependency (Habnauer, New England Journal of Medicine, 334(13), p 841-848, 1996). Aminosalicylates such as sulfasalazine and mesalamine have been used to treat mild or moderately active ulcerative colitis and Crohn's Disease, and to maintain remission (Id at 843). Immunomodulatory drugs such as azathioprine and mercaptopurine have been used in long term treatment for patients with IBD. Common complications with both of these drugs include pancreatitis, which occurs with an incidence of 3-15% of patients, and bone marrow suppression, which requires regular monitoring. More potent immunosuppressive drugs such as cyclosporine and methotrexate have been employed, but toxicity of these drugs limits their use to specific situations of refractory disease states. Other therapeutic approaches include antibiotic therapy and nutritional therapy. Often, therapy involves a combination of the above-described drug therapies in addition to surgical resection of the bowel.
There is no cure for IBD. Ultimately, the chronic and progressive nature of IBD demands a long-term treatment that maximizes the local antiinflammatory effect while minimizing the global systemic effect on the immune system.
Chronic inflammatory disorders such as Crohn's Disease typically demonstrate periods of remission between intervals when the inflammatory is active and requires acute treatment. This is an example of a circumstance wherein it is known beforehand that an individual will develop, or is likely to develop an inflammatory disorder.
Another chronic inflammatory condition believed to be mediated by LTB4 is psoriasis. Psoriasis is a chronic, recurrent, papulosquamous plaque on areas of trauma such as the elbow, knee or scalp, though it may appear elsewhere on the skin. Psoriasis may coexist with lupus erythematosis in some individuals. Current treatments include topical administration of psoralens. “Psoralens” refers to a group of substances found in many different plants, especially psoralea corylifolia. Psoralens interact with nucleic acids and are also used as research tools. Psoriasis is also treated by long-wave ultraviolet radiation. Neither treatment cures or prevents recurrence of psoriasis symptoms.
Another chronic inflammatory disorder believed to be mediated by LTB4 is rheumatoid arthritis, which is an autoimmune disease of the joints. Rheumatoid arthritis is characterized by the following criteria 1-7, wherein criteria 1-4 are present for more than 6 weeks: (1) morning stiffness in and around joints lasting at least one hour before maximum improvement; (2) soft tissue swelling (arthritis) of three or more joints observed by a physician; (3) swelling (arthritis) of the proximal interphalangeal, metacarpal phalangeal, or wrist joints; (4) symmetric swelling; (5) rheumatoid nodules, i.e., a granulomatous lesion characterized by central necrosis encircled by a palisade of monocytes and an exterior mantle of lymphocytic infiltrate. These lesions present as subcutaneous nodules, especially at pressure points such as the elbow in individuals with rheumatoid arthritis or other rheumatoid disorders; (6) presence of rheumatoid factors, i.e., an autoantibody in the serum of individuals with rheumatoid arthritis; and (7) roentgenographic erosions, i.e., joint lesions visible on an X-ray.
Rheumatoid arthritis is a chronic disorder for which there is no known cure. The major goals of treatment of rheumatoid arthritis are to reduce pain and discomfort, prevent deformities and loss of joint function, and maintain a productive and active life. Inflammation must be suppressed and mechanical and structural abnormalities corrected or compensated by assistive devices. Treatment options include reduction of joint stress, physical and occupational therapy, drug therapy, and surgical intervention.
There are three general classes of drugs commonly used in the treatment of rheumatoid arthritis: non-steroidal anti-inflammatory agents (NSAID's), corticosteroids, and remittive agents or disease modifying anti-rheumatic drugs (DMARD's). NSAID's and corticosteroids have a short onset of action while DMARD's can take several weeks or months to demonstrate a clinical effect. DMARD's include leflunomide (Arava™), etanercept (Enbrel™), infliximab (Remicade™), antimalarials, methotrexate, gold salts, sulfasalazine, d-penicillamine, cyclosporin A, cyclophosphamide and azathioprine. Because cartilage damage and bony erosions frequently occur within the first two years, rheumatologists now move more aggressively to a DMARD agent.
Treatment of rheumatoid arthritis by chronic administration of a corticosteroid involves the same side effect profile as discussed regarding IBD above. Chronic administration of NSAID's also produces side effects. The most common toxicity of NSAID's is gastrointestinal disturbance. Because prostaglandins play a role in the regulation of renal blood flow and maintenance of glomerular filtration, NSAID's can impair renal function in certain patients. Weight gain and cushingoid appearance is a frequent problem and source of patient complaints. Recent studies have raised concern over the increased cardiovascular risk and accelerated osteoporosis associated with low dose prednisone particularly at doses above 10 mg daily.
Gout is another inflammatory disorder believed to be mediated by LTB4. Gout is characterized by a disturbance of uric-acid metabolism occurring chiefly in males. Gout is characterized by painful inflammation of the joints, especially of the feet and hands, and arthritic attacks resulting from elevated levels of uric acid in the blood and the deposition of urate crystals around the joints. The condition can become chronic and result in deformity.
Gout can present another circumstance wherein it is known beforehand that an individual will or is likely to develop an inflammatory disorder. In the instance of patients undergoing radiotherapy or chemotherapy, the individual may experience a dramatic rise in serum uric acid levels associated with lysis of the tumor mass. Such large increases in uric acid can deposit urate crystals in synovial fluid of joints thereby causing the inflammatory disorder, gout. When such a rise in serum uric acid levels is known to be likely, prophylaxis with an LTB4 antagonist can act to prevent the inflammatory condition of gout.
Radiation-induced gastrointestinal inflammation is another inflammatory disorder believed to be mediated by LTB4. Radiation works by damaging cancer cells, but unfortunately can damage non-diseased tissue as well, causing a typical inflammatory reaction in response. Therapeutic radiation is thus generally applied to a defined area of the subject's body which contains abnormal proliferative tissue in order to maximize the dose absorbed by the abnormal tissue and minimize the dose absorbed by the nearby normal tissue. However, it is difficult (if not impossible) to selectively administer therapeutic ionizing radiation to the abnormal tissue. Thus, normal tissue proximate to the abnormal tissue is also exposed to potentially damaging doses of ionizing radiation throughout the course of treatment. Moreover, some treatments that require exposure of the subject's entire body to the radiation, in a procedure called “total body irradiation”, or “TBI.” The efficacy of radiotherapeutic techniques in destroying abnormal proliferative cells is therefore necessarily balanced by the associated cytotoxic effects on nearby normal cells.
After or during a course of radiotherapy, LTB4-mediated inflammatory processes may be triggered, causing damage to the bowel, and leading to sloughing of the cells of the inner lining of the GI tract. Radiation-induced gastrointestinal inflammation can present another circumstance wherein it is known beforehand that an individual will or is likely to develop an inflammatory disorder. In the instance of patients undergoing radiotherapy, the inflammation, damage and sloughing of the gastrointestinal tract is a predictable side effect of the radiotherapy.
New antiinflammatory agents are needed which are useful in the treatment of inflammatory disorders such as IBD, rheumatoid arthritis, gout, psoriasis and radiation-induced gastrointestinal inflammation. In particular, agents are needed that are appropriate for chronic long-term use in treatment. In addition, agents are needed that are useful in the prevention of LTB4-mediated inflammatory disorders that occur secondary to observable events such as ionizing radiation therapy.
Thromboxane A2 
Thromboxane A2 (TXA2), like LTB4, is a product of the arachidonic acid metabolic pathway. TXA2 induces a variety of differential cellular responses including platelet aggregation, contraction of vascular and bronchial smooth muscle cells (SMC), potentiation of hypertrophic and mitogenic responses in vascular SMC and endothelial cells.
TXA2 is considered to be an important mediator of asthma because it can induce contraction of airway smooth muscle, and because it has been implicated in airway hyperresponsiveness in animal models wherein increased airway reactivity was induced by allergens, platelet-activating factor (PAF), LTC4, LTD4, LTB4, bradykinin, endothelin, endotoxin and ozone. (See J. Dogne et al., Expert Opin. Investig. Drugs (2002), 11(2), and references cited therein, the entire disclosures of which are incorporated herein by reference.)
TXA2 has also been implicated in the pathophysiology of radicular pain induced by hemeated nucleus pulposis. A study in a rat model examined the role of TXA2 (and LTB4) in the hyperalgesia induced by application of nucleus pulposus to the lumbar nerve root in the rat. A TXA2 synthetase inhibitor, injected into the epidural space, decreased mechanical hyperalgesia at both three and seven days after epidural injection. There were no significant differences in sensitivity to noxious thermal stimuli following application of the nucleus pulposus or an epidural injection. Epidural injection of TXA2 synthetase inhibitor may attenuate the painful radiculopathy due to lumbar disc herniation.
TXA2 has further been implicated as an in vivo mediator of fibroblast growth factor (FGF)-stimulated angiogenesis. See, T. Daniel et al., Cancer Research, 59, 4574-4577, Sep. 15, 1999, the entire disclosure of which is incorporated herein by reference. Thromboxane synthase inhibitors have further been shown to inhibit metastasis of lung carcinoma in a mouse model, thus demonstrating the involvement of TXA2 in angiogenesis and tumor metastasis. See, D. Nie et al., Biochem. Biophys. Res. Commun., 2000, 267(1), p. 245-251, the entire disclosure of which is incorporated herein by reference.
TXA2 is also believed to possess anticoagulant activity. See Schenk et al., “Antiplatelet and anticoagulant effects of “HN-11 500,” a selective thromboxane receptor antagonist,” Thromb. Res. 2001 Jul. 15; 103(2):79-91.
Anticoagulant has potential therapeutic value in chronic inflammation according to a model associating chronic inflammatory disorders with a coagulation protein defect is termed immune system activation of coagulation (ISAC). The model proposes that a majority of individuals diagnosed with certain chronic inflammatory illnesses may, based on clinical criteria, be potentially defined as or involve AntiPhospholipid Antibody Syndrome (APS)-with the endothelial cell (EC) as the disease target. These patients have a hypercoagulable, state demonstrated by increased markers of coagulation activation and increased blood viscosity due to the generation of Soluble Fibrin Monomer (SFM). The CFS/FM process and related processes may be triggered by a variety of pathogens (CMV, HHV6, Mycoplasma, Chlamydia pneumonia, etc.), or some vaccines, resulting in pathogen-mediated immune activation that induces antibodies which cross react with EC protective proteins B2GPI & Annexin V. These antibodies dislodge the protective proteins from EC surfaces, exposing PhosphatidylSerine (PS) on the EC surfaces in capillary beds.
Pathogens induce inflammatory responses which include cytokine modulation of EC to down regulate the antithrombotic environment (ThromboModulin, tPA) in favor of prothrombotic expression of Tissue Factor (TF). TF and PS exposure allows binding of the coagulation tenase and prothombinase complexes to EC surfaces. This results in thrombin generation leading to SFM formation. SFM dimerizes easily, increasing blood viscosity and precipitating out on EC surfaces as fibrin(oid) deposition, creating local ischemia and pathology, blocking nutrient and oxygen delivery in the microcirculation. A blood clot does not form because there is not enough of a thrombin burst to activate Factor XIII to cross link the fibrin into a clot.
A hereditary defect in a coagulation regulatory protein; such as protein C, protein S. Factor VL, prothrombin gene mutation, Heparin Cofactor II, tPA, PAI-1, Lp(a), or elevated Factor II, X, XII, or homacysteine is predispositional in greater than 75% of patients. Because this hypercoagulability does not result in an immediate thrombosis (100% occlusion), but rather in fibrin deposition (50-95%), it has been suggested that an appropriate name for this antiphospholipid antibody process would be Immune System Activation of Coagulation (ISAC) syndrome.
The ISAC model provides an explanation for the therapeutic benefits reported with low dose anticoagulant therapy (heparin or warfarin) in some of these patients. Diagnoses with published associations include: Chronic Fatigue. Syndrome/Fibromyalgia (CFS/FM), Infertility (Recurrent Fetal Loss and Fetal Wastage Syndromes), Osteonecrosis of the Jaw, Multiple Sclerosis (MS), Depression and Autism. Diagnoses under investigation include: Crohn's Disease and Inflammatory Bowel Disease (IBD), Late Lyme Disease, Sjogren's Syndrome (SS), Transient Ischemic Attack (TIA), Attention Deficit Disorder (ADD) and Parkinson's Disease. See Berg et al., “Chronic Fatigue Syndrome &/or Fibromyalgia as a variation of antiphospholipid antibody syndrome (APS): An explanatory model and approach to laboratory diagnosis,” Blood Coagulation and Fibrinolysis, 1999, 10:435-438.
New TXA2 agents are needed which may be useful in the treatment of TXA2-mediated disorders such as asthma, pain, tumors in which angiogenesis associated with the tumor is mediated by TXA2, and in chronic inflammatory illnesses such as, for example Chronic Fatigue Syndrome/Fibromyalgia, IBD, Crohn's Disease, late Lyme disease and IBD.
Adenosine
Adenosine is a multi-purpose signal molecule that regulates a variety of cellular functions and is released under conditions of physiological stress. The actions of adenosine are mediated through four receptor subtypes (A1, A2A, A2B and A3).
Adenosine acts at the A1 receptor subtype to cause decreases in heart rate, force of contraction, and responsiveness to adrenaline, and at the A2A receptor subtype to cause dilation of coronary arteries to enhance blood flow to the heart. In the central nervous system (CNS), adenosine, released during episodes of epilepsy or as a consequence of hypoxia or stroke, acts at the A1 receptor subtype to exert a neuroprotective action by decreasing electrical excitability, inhibiting the release of excitatory amino acids (EAA) and acts at the A2A receptor subtype to increase cerebral blood flow.
In the kidney, A1 receptors located on preglomerular vessels and in the tubule are involved in the regulation of glomerular filtration. Whole body fluid balance is strongly dependent on the ability of the kidney to maintain stable glomerular filtration. Several antagonists to A1 receptors have been developed. These agents generate excess fluid (diuresis) and sodium (natriuresis) excretion in control animals and animal models of fluid retention, as well as in normal and oedematous humans. In both animals and humans, these effects are generally achieved without major changes in glomerular filtration. Animal studies have confirmed the location of A1 receptors in relevant tissue sites in the kidney. More highly selective antagonists for A1 receptors are regularly developed, improving their use in fluid retaining disorders. See Welch W J, “Adenosine type 1 receptor antagonists in fluid retaining disorders,” Expert Opin Investig Drugs 2002 November; 11(11):1553-62.
Adenosine, whether endogenously released or added exogenously, is a potent antiinflammatory agent. Adenosine mediates its antiinflammatory effects via interaction with specific receptors (A1, A2a, A2b and A3) on the surface of inflammatory cells. Receptor-specific analogs of adenosine have been shown to increase the rate at which wounds heal. Am J Pathol 2002 June; 160(6):2009-18.
Adenosine promotes wound healing and mediates angiogenesis in response to tissue injury via occupancy of A2A receptors. See Montesinos et al., “Adenosine promotes wound healing and mediates angiogenesis in response to tissue injury via occupancy of A2A receptors,” Am. J. Pathol. 2002 June; 160(6):2009-18, and Victor-Vega et al., “Adenosine A2A receptor agonists promote more rapid wound healing than recombinant human platelet-derived growth factor (Becaplermin gel),” Inflammation 2002 February; 26(1):19-24.
Adenosine is believed to mediate gastrointestinal relaxation through two different inhibitory receptor subtypes; A1 receptors on the enteric neuron and A2B receptor on the smooth muscle in the guinea-pig distal colon. See Kadowaki et al., “Molecular identification and pharmacological characterization of adenosine receptors in the guinea-pig colon,” Br. J. Pharmacol. 2000 March; 129(5):871-6.
Adenosine binds to the receptor subtypes and activates the receptors to produce G proteins. G proteins themselves can either stimulate (Gs) or inhibit (Gi) the enzyme adenylate cyclase so as to generate or prevent the manufacture of cyclic AMP. In addition, G coupled proteins can open potassium channels in cardiac tissue, resulting in depression of cardiac electrical activity. The following Table enumerates adenosine activity on different tissues.
TABLE 1Effect of Adenosine Receptor ActivationReceptorSubtypeHeartCNSKidneyOtherA1Heart Rhythm-Wakefulness-Anti-Anti-lipolyticdecrease indecrease indiuresisinsulin enhancerheart rate,electricalAntihypertensiveforce of atrialexcitabilityWound healingcontraction,and inhibitionHair growthandof excitatoryresponsivenessamino acidto adrenaline(EAA) releaseA2ARegulatesAnti-Wound healingblood vesselInflammatory-tone-dilationcerebral bloodof theflow increasecoronaryarteriessupplyingbloodto the heartmuscleA2BAllergicResponses GITract RelaxationAnti-InflammatoryA3Cardio-AllergicprotectiveResponses
Adenosine helps protect the heart muscle from damage when myocardial ischemia occurs. See Maddock H L et al., “Adenosine A3 receptor activation protects the myocardium from reperfusion/reoxygenation injury,” Am. J. Physiol. Heart. Circ. Physiol. 2002 October; 283(4):H1307-13. When this happens, adenosine is released in the heart vessels and myocardium and acts to: enlarge vessels to increase blood and oxygen supply; improve energy supply for the myocardium and decrease energy needs; produces angina pectoris, the signature warning symptom of myocardial ischemia. Adenosine has a depressant effect on sinoatrial node activity and thus exerts an arrythmogenic effect. See Belhassen B., “Adenosine triphosphate in cardiac arrhythmias: from therapeutic to diagnostic use,” Pacing Clin. Electrophysiol. 2002 January; 25(1):98-102; and Meester, B J et al., “Pharmacological classification of adenosine receptors in the sinoatrial and atrioventricular nodes of the guinea-pig,” Br. J. Pharmacol. 1998 June; 124(4):685-92. This makes adenosine effective in treating tachyarrhythmias involving the sinoatrial node. Ongoing research suggests that adenosine will be important in protecting the heart during open-heart surgery. See Safran N et al., “Cardioprotective effects of adenosine A1 and A3 receptor activation during hypoxia in isolated rat cardiac myocytes,” Mol. Cell. Biochem. 2001 January; 217(1-2): 143-52.
Adenosine signaling has also been implicated to play a role in various types of lung inflammation including those seen in asthma and chronic obstructive pulmonary disease (COPD). Asthma is an inflammatory disease of the lung characterized by acute nonspecific airway hyperreactivity in association with chronic pulmonary inflammation. The disease effects approximately 10% of children and 6% of adults in the United States alone, and its incidence is increasing at an alarming rate. COPD is a progressive disease process that most commonly results from smoking. COPD is characterized by difficulty breathing, wheezing and a chronic cough.
The major observations are that adenosine levels are elevated in the lungs of asthmatics, inhaled adenosine causes bronchoconstriction in asthmatics but not normal subjects, the pattern of adenosine receptor expression is altered in the lung of asthmatics, and theophylline, an adenosine receptor antagonist, has well recognized benefits in the treatment of asthma. In addition to this clinical evidence, there are many in vitro studies in both human and animal cells that implicate adenosine as a modulator of inflammatory processes that are central to asthma. Most notable are adenosine's ability to enhance mediator release from mast cells, and to influence eosinophil survival and chemotaxis. However, despite these lines of evidence, a clear implication for adenosine signaling in asthma, and the cell types and mechanisms involved, are unclear.
Adenosine is also a modulator of dopamine mediated motor responses. A new therapeutic approach to the treatment of Parkinson's disease is to synergistically modulate the effects of dopamine agonists in order to decrease the dosages of drugs used. Recent research suggests a potential positive role of A2A antagonists in Parkinson's disease. See “Adenosine Receptors and Parkinson's Disease,” Hiroshi Kase (Editor), Peter J. Richardson (Editor), Peter Jenner (Editor), Academic Press; ISBN: 0124004059; 1st edition (Jan. 15, 2000). The current chronic treatment of Parkinson's disease with L-DOPA causes important complication as motor fluctuations and dyskinesias, thus new agents are needed that adequately treat Parkinson's Disease without concomitant dyskinetic side effects.
Adenosine has also been investigated as a potential mediator in regeneration of hematopoietic progenitor cells in mouse models of severe myelosuppression. In the study, drugs that elevate extracellular adenosine were shown to modulate regeneration from severe myelosuppression resulting from combined exposure of the animals to ionizing radiation and carboplatin. In the model, elevation of extracellular adenosine was induced by joint administration of dipyridamole (DP), a drug inhibiting the cellular uptake of adenosine, and adenosine monophosphate (AMP), serving as an adenosine prodrug. The test drugs were administered in a 4-day treatment regimen starting on day 3 after induction of myelosuppression. (The drug regimen was tested with and without co-administration of granulocyte colony stimulating factor.) The effects of the drug treatments on progenitor cells were reflected in the peripheral blood in later time intervals of days 15 and 20 after induction of myelosuppression, especially as significantly elevated numbers of granulocytes and less pronounced elevation of lymphocytes and erythrocytes. The results substantiate the potential of drugs elevating extracellular adenosine for clinical utilization in myelosuppressive states, e.g. those accompanying oncological radio- and chemotherapy.
New agents are needed that selectively act at specific adenosine receptors and may thus which be useful in the treatment of adenosine-mediated disorders. Such disorders include, for example, neurological disorders such as epilepsy, stroke and cerebral ischemia; heart failure; and regeneration of hematopoietic cells associated with myelosuppression caused by ionizing radiation therapy of cancer chemotherapy.